1. Field
The present invention relates to a functionalized diene elastomer composed of a specific functionalized block copolymer, of a specific star-branched block copolymer and of a content of less than 15% by weight of a specific non-tin-functional block copolymer.
2. Description of Related Art
The reduction in the hysteresis of the mixtures is an ongoing objective of the tire industry in order to limit the consumption of petrol and to thus protect the environment. This reduction in hysteresis must, however, be carried out while keeping intact, indeed even while improving, the processability of the mixtures.
Many solutions have already been experimented with in order to achieve the objective of fall in hysteresis. In particular, the functionalization of the polymers by a functional group which interacts with the reinforcing filler has appeared as an advantageous route.
Functional groups which interact with the filler have thus been attached at the chain end at the start or end of polymerization by means of functional initiators or functionalization agents. By way of example, 4,4′-bis(diethylamino)benzophenone, also known as DEAB, or other aminated functional groups which interact with carbon black have been added at the end of polymerization, as described in the patent documents FR 2 526 030 and U.S. Pat. No. 4,848,511. The polymers coupled by or star-branched by tin comprise functional groups capable of interacting with carbon black which are introduced at the end of polymerization. Mention may be made, by way of example, of the European patent document EP 0 709 235. Functional groups which interact with silica have also been attached at the chain end at the end of polymerization, such as functional groups comprising a silanol group which are disclosed in the patent document FR 2 740 778 or functional groups comprising alkoxysilane or aryloxysilane groups which are described in the document U.S. Pat. No. 5,066,721. The majority of these solutions, both for the black and for the silica, genuinely result in a limitation on hysteresis of the corresponding compositions but concomitantly in a greater difficulty in processing these same compositions.
Patent EP 1 278 789 describes a copolymer comprising n blocks (n=2 or 3) intended to form an elastomer matrix of a crosslinkable rubber composition, each of the said blocks comprising an essentially unsaturated diene elastomer and one or each of the said blocks forming a chain end of the said copolymer being composed of a polyisoprene. The number-average molecular weight of the polyisoprene block is between 2 500 and 20 000 g/mol and the number-average molecular weight of the block of the copolymer which is other than the said polyisoprene block is substantially between 80 000 g/mol and 350 000 g/mol. The use of the said block copolymer makes it possible to significantly optimise the results of reduction in hysteresis and processability for the said rubber composition in which it is present.
It is also known that polymers having narrow molecular distributions confer a reduced hysteresis on the rubber compositions in which they are present. In particular, linear functional diene elastomers with narrow molecular distributions before functionalization exhibit improved hysteresis properties. However, these elastomers exhibit an increased cold flow in comparison with the same elastomers exhibiting a broad molecular distribution before functionalization, which is damaging for the storage and transportation of the elastomers.
Many solutions exist in order to reduce the cold flow. The work Nouvelles Recherches dans le Domaine des Composés Macromoléculaires [New Research in the Field of Macromolecular Compounds], editor(s) Ceausescu E, Oxford, Pergamon Press Ltd., 1984, pp. 343-56, 72, mentions methods for reducing cold flow, such as the increase in the molecular weight, the star-branching or the degree of branching. However, the increase in the molecular weight is highly damaging to the use of the mixtures while the branching is accompanied by an increase in the hysteresis of the mixtures. Furthermore, surprisingly, the polymers solely star-branched by tin (3 or 4 branches) exhibit greater hysteresis in comparison with the polymers coupled by tin.
It is also known that, in order to solve the problem of cold flow, the polydispersity index of the elastomer can be increased. However, this is not without effect on the properties of the rubber compositions in which it is present, in particular on the hysteresis or the processability of the mixtures, for example.